A source field plate is known as an electric field relaxation technique for a field effect transistor (FET). The source field plate arranged between a gate and a drain is capable of performing relaxation of electric field concentration near a gate electrode, and as a result, an FET improves a withstanding voltage thereof. In order to realize the relaxation of the electric field effectively, a source field plate is arranged near the gate electrode, or is arranged overlapping with the gate electrode.
As for an FET which operates at high frequency, such as a millimeter wave band, a high gain is expected when it operates at relatively low frequency, such as a microwave band etc. On the other hand, suppression of oscillation is difficult because feedback becomes large. Although the source field plate is effective in suppression of oscillation, the source field plate increases a capacitance Cgs between a gate and a source because the source field plate short-circuited to the source is arranged near the gate. As a result, gain of the FET decreases.